1. Field of the Invention
The present invention relates to a process for the production of a multidetector and also relates to the multidetector obtained by this process.
The multidetectors in question are essentially multidetectors with ionization chambers. They are of the type used in X-ray tomoscanners. A multidetector with ionization chambers has a plurality of adjacent chambers, which are separated from one another by metallized partitions forming an electrode: alternatively an anode or cathode. All these chambers are filled with an ionizable gas (e.g. xenon). When the gas contained in a chamber is excited by X-radiation, it is ionized and consequently gives rise to an ionization current between the electrodes on either side of said chamber. The intensity of the ionization current reveals the intensity of the exciting X-radiation. On knowing the intensity of an emitted X-radiation, it is possible to deduce therefrom the radiological absorption density of a zone of a medium traversed by part of the radiation. For this purpose, after traversing said medium, the intensity of said radiation part is measured when it arrives at a chamber level with said zone. The adjacent arrangements of the multidetector chambers make it possible to draw up a cartography of the radiological absorption densities of the adjacent zones of the examined medium.
On of the most important factors of the ionization current intensity is the width of a chamber, the distance separating the two electrodes or partitions of said chamber. In order to render homogeneous the ionization current measurements in all the chambers, it is appropriate to produce said chambers with equal widths.
To this end, use is made of ceramic supports, in which adjacent grooves or slots are produced by sawing. The partitions are then slide one by one into said slots for defining the chambers. This process suffers from a disadvantage, namely that in order to be able to slide the partitions/electrodes into the slots, the latter must be sufficiently overdimensioned with respect to the thicknesses of said partitions. This necessary overdimensioning is prejudicial to the accuracy of the fitting of the partitions.
2. Description of the Prior Art
In the present state of the art, tomoscanners are provided with a multidetector having approximately 1000 ionization chambers. When distributed over approximately 1 meter and once the thickness of the partitions has been deduced, the width of the chambers is approximately 0.5 mm. If it is wished to obtain a good homogenereity of the width of the chambers, it is advantageous if said width does not vary by more or less than 10%. Therefore the sawing of the slots must take place with an accuracy equal to or greater than 5% of the width of a chamber (twice 5%=10%, because there are two partitions per chamber). This leads to machining tolerances of approximately 20 microns, which require costly equipment.
Moreover, it is not possible to produce ceramic supports with a large size. Generally in order to obtain a multidetector with a length of approximately one meter, it is necessary to align end to end 3 segments of approximately 300 mm each. The installation of these segments is difficult, in view of the fact that the organization system of the chambers continues, even on passing from one segment to another. Therefore the assembly tolerance of the individual segments must have the same accuracy of that required for the machining of the slots. It is standard practice for operators to use a magnifying glass to adjust said assembly of segments. It is also impossible to bond the ends of said segments to one another. Thus, bonding exerts forces on the bonded pieces, so that it is not possible to control the spacing of the thus approached pieces. The production of the partition supports is further complicated by the fact that it is necessary to provide guard rings. The latter, which are metal strips arranged orthogonally to the partitions in the support, are used for intercepting parasitic ionization currents. In the present state of the art, each segment of the support is itself formed from four elongated elements, which are joined to one another. Prior to the joining of these four elements, there backs are covered with a coating of silver varnish, which acts as a conductive electrode and serves as the guard ring. Machining of the slots is only carried out on a segment after the four elements forming it have been joined by bonding. It is pointed out that this difficult slotting operation must be carried out the same number of times as there are multidetectors to be constructed.